建一个XMLHttpRequest对象池

            {
                // 加随机数防止缓存
                if (url.indexOf("?") > 0)
                {
                    url += "&randnum=" + Math.random();
                }
                else
                {
                    url += "?randnum=" + Math.random();
                }

                open(method, url, true);

                // 设定请求编码方式
                setRequestHeader(''Content-Type'', ''application/x-www-form-urlencoded; charset=UTF-8'');
                send(data);
                onreadystatechange = function ()
                {
                    if (objXMLHttp.readyState == 4 && (objXMLHttp.status == 200 || objXMLHttp.status == 304))
                    {
                        callback(objXMLHttp);
                    }
                }
            }
            catch(e)
            {
   &